Recently, a high solid coating composition comprising crosslinked polymeric microparticles dispersed in a conventional coating composition containing film-forming resinous vehicle, has been watched with keen interest in various coating areas, and especially in an automobile industry, because of the eminent workability and capability of resulting a coating with excellent film performance.
These particles are composed of polymer crosslinked to the extent that it is insoluble in an organic solvent in which the particles are dispersed, finely pulverized to micron size, and stably dispersed in the coating composition. Dispersion of said microparticles in a solvent or a carrying vehicle is sometimes called, for convenience, as "microgels".
In preparing such microgels, various methods have been proposed. One of the methods proposed comprises the combination of steps of preparing fine particles of polymer by emulsion polymerization technique from ethylenically unsaturated monomer and other crosslinkable, polymerizable monomer in an aqueous medium, and separating thus formed microgel particles from the reaction system by solvent replacement, azeotropic distillation, centrifugal separation, filtering, drying and other means. Another one is the so-called NAD (non-aqueous dispersion) method comprising reacting monomer having an ethylenical unsaturation and other copolymerizable crosslinking monomer in a non-aqueous organic solvent which may dissolve monomers but not the formed polymer, and separating thus formed polymer fine particles therefrom.
For industrial purposes, much preference is given to the former because the formation of fine particles and the removal of reaction heat can be easily done. However, in that method, one extra step is always required to remove water from the formed emulsion. Furthermore, since a comparatively low molecular compound, mainly of anionic or cationic surfactant, is customarily used as an emulsifier for assisting effective dispersion of monomers in an aqueous medium and this is always carried, by adhesion, on the surfaces of polymer particles, the same is, when coated, necessarily contained in the coating, exerting harmful effects on the film performance with respect to water resistance and the like. In addition, in the emulsion polymerization step, an aqueous medium is used, whereas in the formulation of coating composition, (a) volatile organic solvent (s) as aromatic hydrocarbons, is(are) customarily used. Since it is quite difficult to use the emulsifier already used in the emulsion polymerization step for for assistance in the dispersion of monomers in an aqueous medium, direct use of microparticles in the subsequent formulation step to obtain a stable dispersion thereof in a different environment i.e., an organic solvent, requires special steps to obtain the stable dispersion of said microparticles in the coating composition. On the other hand, in the NAD method, use is made of a non-aqueous organic solvent which will scarcely dissolve the polymer in it. Therefore, the thus formed product may be directly added to the coating composition. Or even if the separation of microparticles is to be desired, it may be easily done by mere adoption of simple filtering means. However, this NAD method introduces various problems inherent thereto and differing from those of the emulsion polymerization method. First of all, this method requires a particular type of dispersion stabilizing agent in the copolymerization of ethylenically unsaturated monomer and crosslinkable, copolymerizable monomer in a non-aqueous organic medium to obtain microgels. Usually, said stabilizing agent is a graft copolymer comprising a polymer backbone which is non-solvatable by the reaction medium of organic liquid, and a plurality of solvatable polymer chains pendant from the backbone. However, actual selection of said graft copolymer has to be made very carefully in due consideration of affinity for the polymer particles as well as affinity for the solvent, depending on the types of monomers and of the non-aqueous organic solvent used. Furthermore, since a low polarity liquid solvent mainly composed of aliphatic hydrocarbons is used as an organic liquid in the preparation of microgels and a relatively highly polar organic solvent is used in the formulation of coating composition, the dispersion stabilizing agent used in the microgel formation step is almost useless for the stabilization of microgels in the coating composition. As a solution to this problem, laid Open Japanese Patent Application Nos. 133234/78, 133235/78, 133236/78, and 150439/79 suggest that after formation of microgels, particular monomers capable of forming polymer having the same composition as that of the film-forming polymer used in the coating composition are polymerized on the surfaces of said microgel particles, thereby effecting modification of polymer surfaces so as to accomodate themselves to the different circumstances. Thus, there are various problems in heretofore known methods for the preparation of microgels and high solid coating compositions using the same and no satisfactory solutions have been found yet.
Under the circumstances, it would be an immeasurable advancement of the technical level concerned if one could provide microgels accompanied by no harmful component and capable of admixing with a coating composition without the necessity of adopting any special after treatment or using a particular type of stabilizing agent.